Bacterial endotoxins or lipopolysaccharides (LPS) and some polynucleotides (polyinosinate, polyuridylate, etc.) act as mitogens for bone-marrow derived (B) lymphocytes. The present investigation was initiated in order to find out the mechanism of activation of B- lymphocytes by these mitogens. Preliminary results indicate that there is a difference in the mode of activation between B-lymphocytes and thymus-derived or T-lymphocytes. While a shorter presence of mitogens in culture is required for the maximum stimulation of T-lymphocytes, a prolonged presence is necessary for the activation of B-cells by mitogens. Binding experiments with radiolabelled mitogens C14-LPS and H3-polyinosinate (poly I) suggest that lymphocytes bind these mitogens. B-cells appear to bind more mitogens as compared to T-cells. Further investigation on the mechanism of binding is in progress.